I’ll bet that egg has two yolks. It’s not unusual for a hen to occasionally have a glitch in her egg making factory and lay one of those, especially pullets just starting to lay.
Wyorp, consider 25 pullets just starting to lay eating a liter of oyster shell a day. I don’t think my rabbit feeder holds that much. That is an unbelievable amount of oyster shell. It would be interesting to put a game camera on that and see what is taking the oyster shell. I just can’t see it being 25 pullets.
I also would not restrict them to once or twice a week. They need it for their egg shells more often than that. I just can’t believe it’s the chickens eating that much.